"There are a lot of people in this country who are very confused ... There's a real need here in my view for clarity in terms of what actually can be accomplished," Jim Webb, a Democratic senator, said at the committee hearing.
Richard Lugar, the most senior Republican on the senate committee and one of the party's most respected elder statesmen on international affairs, said: "There's substantial concern about our course in Afghanistan."
He cautioned that it is "unrealistic to expect that a significant downsizing of US forces could occur at that time [July 2011] without security consequences".
For his part, Richard Holbrooke, the US special representative for Afghanistan and Pakistan, defended the administration's approach, saying that Obama's decision to send 30,000 more troops into the fight was matched by a three-fold increase in civilian aid workers helping Afghanistan improve governance and its own security forces.
He said that Obama's July 2011 target date was merely to begin withdrawals and that any reduction in forces would be dictated by the conditions on the ground.
"I do not want to give an optimism/pessimism report to you," Holbrooke said.
"I think there are significant elements of movement forward in many areas but I do not yet see a definitive turning point in any direction."
Jim DeMint, a Republican senator, joined his colleagues from both political parties to say Obama's goal to begin withdrawing US troops in July 2011 suggested that the US was not committed to victory in Afghanistan.
"The deadline is defeating us. People know we're leaving," he said.
Noting that Afghanistan last month surpassed Vietnam as the longest military campaign in US history, John Kerry, the committee chairman, said civilian reconstruction efforts in the south and east of the country were off to a slow start while corruption appeared to be growing.
"Many people are asking whether we have the right strategy. Some suggest this is a lost cause," Kerry, a Democrat, said in his opening statement.
"This is the time to ask hard questions about the progress we are making toward our objectives of defeating al-Qaeda and bringing a measure of stability to Afghanistan."
The senate hearing came as Hillary Clinton, the US secretary of state, prepares to lead a US team to a conference in Kabul, the Afghan capital, to try to put more detail on plans to gradually cede responsibility to the government of Hamid Karzai, the president.
Critics have called Karzai an unreliable partner in the fight to defeat the Taliban, but Holbrooke suggested that the Afghan leader's efforts to woo Taliban soldiers off the battlefield could result in an eventual political solution to the conflict.
The debate over the Afghan war comes as Obama's Democratic Party prepares for tough congressional elections in November amid public frustration over the slow economic recovery and seemingly intractable Afghan conflict.
The US has lost more than 1,000 soldiers in the war and casualties are mounting. More than 100 US and other Nato-led troops were killed in Afghanistan in June, the bloodiest month of the war.